Patients with heart diseases, in so far as is medically necessary and expedient, are subjected to scans using X-ray computer tomographs in order to examine the heart, a scan being intended to mean recording a multiplicity of 2D X-ray projections of the heart from different projection directions, usually with an incremental advance of the heart or the patient relative to the X-ray recording system of the X-ray computer tomograph. The purpose of the examination is to generate high-quality and informative images of the heart, which often form the basis for a diagnosis.
Since the heart is a moving organ, when reconstructing slice images and 3D images of the heart, which is done on the basis of the recorded 2D X-ray projections of the heart, attempts are made to use only those 2D X-ray projections that have been recorded in the cardiac phase of the patient's cardiac cycle in which the heart has performed almost no movement, particularly in order to avoid motion artifacts in the reconstructed slice images and 3D images of the heart. In order to determine the cardiac cycle of the patient's heart, it is usual to record an electrocardiogram (ECG) of the patient's heart.
For producing slice images and 3D images of the heart, 2D X-ray projections of the heart are recorded over several cardiac cycles with parallel recording of the electrocardiogram, and subsequently only the 2D X-ray projections suitable for the reconstruction are selected on the basis of the electrocardiogram, for which reason this is also referred to as a retrospective method.
In an alternative procedure, 2D X-ray projections of the heart are likewise obtained over several cardiac cycles, but on the basis of an electrocardiogram recorded in parallel only if the heart is in a cardiac phase for which it performs almost no movement. This procedure has the advantage that the patient is exposed to a lower X-ray dose.
Obtaining 2D X-ray projections of the heart over several cardiac cycles has been or is necessary since not enough 2D X-ray projections from different projection directions of the patient's heart could be obtained with the previously available X-ray computer tomographs within only one resting phase of the patient's cardiac cycle, in order to reconstruct high-quality slice images and 3D images of the heart. The reason for this resides in the limited rotation speed of the gantry rotating around the patient and comprising the X-ray system, as well as the limited acceleration and adjustment speed of the patient table supporting the patient.
Recently, however, X-ray computer tomographs, in particular X-ray computer tomographs having two X-ray systems arranged on a gantry and offset by about 90°, have become available, with which it is possible to obtain enough 2D X-ray projections from different projection directions within the resting phase of only one cardiac cycle of a patient's heart, so that high-quality slice images and 3D images of the patient's heart can be reconstructed. Patients who are viable for a successful examination using such X-ray computer tomographs, however, are subject to certain physiological restrictions, particularly in relation to the duration of the cardiac cycle and the tolerable variance of the cardiac cycle.